Hybridoma technology has been utilized to produce monoclonal antibodies to the structural proteins of herpesvirus saimiri. These antibodies have been used to analyze virus structure and replication, and by utilizing both electrophoresis and immunofluorescence, it has been possible to categorize various virus polypeptides into membrane glycoproteins, nuclear antigens, and non-glycosylated structural proteins. Lentiviruses, a groups of retroviruses which cause various diseases, including cancer, in domestic animals, have also been analyzed. Radioimmunoassays to the major structural proteins of several of these viruses have been used to investigate the genetic relatedness between different strains of virus and to provide sensitive assays to assess the virological status of tissues from diseased and experimentally inoculated animals. We have shown that sheep pulmonary adenomatosis virus (SPAV) is closely related to the caprine arthritis-encephalitis virus (CAEV) and to progressive pneumonia virus (PPV) of sheep. Furthermore, equine infectious anemia virus (EIAV) was shown, for the first time, to be distantly related to the sheep and goat retroviruses, CAEV and PPV.